Dive Dry with Dr. Bill

#176: High Density Development in Avalon

Anyone who has looked at some of the development concepts for Catalina that were put together in the 1960's should be very happy that the Wrigley family created the Conservancy a decade later. Although visionary for its time, the Periera Plan called for high density development in Avalon and Two Harbors area, as well as lower density development in the interior complete with a monorail transportation system. Total island population was to reach 20,000 people. Thanks to input from folks like Doug Propst and Dr. Bob Given, Philip K. and Helen Wrigley and Dorothy Wrigley Offield saw the wisdom of protecting the natural ecosystems of the island they loved.

I recently investigated a high density development I discovered right here on Avalon's beaches. I'm sure this construction would never be approved by the City of Avalon's Planning Department (right, Pete?). Hmmm... on second thought, it didn't require an SCE water permit to build. I'm referring to colonies of bryozoa, a small filter feeding invertebrate, on blades of giant kelp washed ashore by our last storm. There must have been thousands, if not tens of thousands (maybe even "billions!") of these tiny critters on a single blade of kelp less than a foot long. If you look at the images accompanying this article, you can get a feel for what I'm talking about!

The bryozoan Membranipora membranacea was one of the first species I identified back in my undergraduate days at Harvard. This species was first described by Linnaeus, the father of "modern" biological classification, in 1767 (slightly before my time). It is occasionally referred to as the sea mat or lacy bryozoan, names which are a lot easier to say than their scientific name. Technically, the old phylum Bryozoa was split after I graduated into the Ectoprocts and Entoprocts, with this species belonging to the former group. To further confuse things, the Ectoprocts are often called by the former phylum name, Bryozoa.

It is commonly stated that this species is known from Alaska to California, or from the Atlantic Ocean, or that it is actually a fairly cosmopolitan species. How else would I know it from Massachusetts! It is also found in Great Britain, Europe and other regions of the world where large kelps (and therefore cooler water) exist. More recent studies suggest that the individuals found in the Pacific Ocean are actually Membranipora tuberculata, a different species from that of the East Coast and Europe. In fact it is now believed that at least three different species in this genus colonize giant kelp.

The zoecium, or home, of an individual zoid measures about 0.4mm (a sixth of an inch for my metrically challenged readers) up to 0.8mm, about the size of the head of a pin... and I'm not referring to a bowling pin. Each individual lives for about eight weeks. Their growth pattern is dependent on their success in the intense competition for space between individuals in the colony. The general pattern is one of radial growth with branching groups of zoids growing outwards from the center. Colonies may reach well over an inch in diameter. When increasing the size of the colony, reproduction is done asexually by budding.

Individual zoids have a U-shaped gut. Their mouth is located inside a nearly circular feeding structure known as a lophophore, and the anus is outside (hence "ecto," and "proct" as in proctologist). This structure has tentacles which feed on bacteria, plankton and organic matter passing by the animal's home or zoecium. In turn they are eaten by nudibranchs, some of which closely resemble the bryozoan colonies themselves. The zoid's tiny size means they can absorb oxygen and pass off carbon dioxide directly, so they do not need gills to breathe. Most individuals serve as feeding zoids known as autozoids. Others may perform specialized functions like defense, reproduction, brooding of eggs (in some species) or anchoring the colony to the kelp blade or other substrate.

These bryozoa are epiphytic, which simply means they form their colonies on top of algae like giant kelp. However, my readers all know that kelp grows at a very fast rate... up to 1-2 feet per day under optimal conditions. A rapidly growing kelp blade is about as stable a surface to attach to as a landmass in an active earthquake zone! To ensure they get the proper "permits" for development, epiphytic organisms usually wait until the kelp blade matures and growth slows before attaching. The bryozoa themselves are pretty "fast" growers... with colonies expanding at the rate of a 0.01" per day or more. They could probably use some help from Pete Edwards and Fine Line Construction in that respect! Eventually these bryozoan colonies may add 2 to 20 pounds of weight for every square yard of blade surface!

These tiny critters are simultaneous hermaphrodites. This means they have both sex organs functioning at the same time. They reproduce sexually to produce larvae that can disperse and colonize new areas. The eggs are produced and then fertilized internally by sperm released through two of their tentacles. Once released, the eggs develop in the plankton, hatching into planktonic larvae that drift for some time and may be very abundant. Although they often self-fertilize themselves, sperm may drift to other individuals and even other colonies so cross-fertilization does occur. Once the larvae arrive at a new location, they settle and begin asexual budding to form the new colony.

Small colonies apparently communicate through electrical signals. Large colonies have runner-like stolons which bud off new zoids. The interactions between colonies run the gamut from cooperation to aggression, and these stolon-based zoids are responsible for defending against aggression from adjacent colonies. So even these tiny critters may engage in warfare with one another. Sound familiar? Can't we all just get along?

© 2006 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, "or "Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Bryozoan colonies on a kelp blade, each with hundreds to thousands of individuals; increasingly
magnified images of these bryozoan colonies through two different microscopes.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia